Stimulation procedures often require the use of well treating materials having high compressive strength. In hydraulic fracturing, such materials must further be capable of enhancing the production of fluids and natural gas from low permeability formations. In a typical hydraulic fracturing treatment, fracturing treatment fluid containing a solid proppant is injected into the wellbore at high pressures. Once natural reservoir pressures are exceeded, the fluid induces fractures in the formation and proppant is deposited in the fracture, where it remains after the treatment is completed. The proppant serves to hold the fracture open, thereby enhancing the ability of fluids to migrate from the formation to the wellbore through the fracture. Because fractured well productivity depends on the ability of a fracture to conduct fluids from a formation to a wellbore, fracture conductivity is an important parameter in determining the degree of success of a hydraulic fracturing treatment. Choosing a proppant is critical to the success of well stimulation.
Proppants used in the art include sand, glass beads, walnut hulls, and metal shot as well as resin-coated sands, intermediate strength ceramics, and sintered bauxite; each employed for their ability to cost effectively withstand the respective reservoir closure stress environment. The relative strength of these various materials increases with their corresponding apparent specific gravity (ASG), typically ranging from 2.65 for sands to 3.6 for sintered bauxite. Unfortunately, increasing ASG leads directly to increasing degree of difficulty with proppant transport and reduced propped fracture volume, thereby reducing fracture conductivity.
More recently, ultra lightweight (ULW) materials have been used as proppants since they reduce the fluid velocity required to maintain proppant transport within the fracture, which, in turn, provides for a greater amount of the created fracture area to be propped. Exemplary of such proppants are significantly lighter deformable particles. Such ULW proppants, like conventional heavier proppants, have the capability to effectively withstand reservoir closure stress environments while increasing fracture conductivity.
Aggregate compositions employing ULW proppants comprised of solid particulates encased within a polymeric coating or continuous phase have been reported. Further useful in many instances are glass bubbles encased within a ceramic continuous phase.
Materials of various specific gravities may be used as the particulates within the aggregate to achieve the desired particle specific gravity and downhole conditions. For example, successful deformable particles include modified ground walnut hulls which are capable of withstanding higher closure stress than walnut hulls in their natural state. Modified walnut hull based ULW proppants are manufactured by impregnating closely sized walnut particles (i.e. 20/30 US mesh) with epoxy or other resins. These impregnated walnut hull particles are then coated with phenolic or other resins. Such walnut hull based ULW proppants have a bulk density of 0.85 grams/cc. Further exemplary of deformable particles are polystyrene divinylbenzene (PSDVB) deformable beads.
In addition to having low specific gravity, ULW proppants must also be of sufficient strength to withstand the rigors of high temperatures and high stresses downhole. ULW proppants, while offering excellent compressive strength, readily soften and loose their compressive strength especially at high temperature and high pressure conditions. For instance, resinous materials currently being utilized as ULW proppants have been observed to deform at elevated temperatures to the extent that under a 5,000 psi stress load at temperatures greater than 250° F., the permeability of the ULW proppant pack is deformed beyond the limits of its commercial utility even though the melting point of the resin is at a temperature of well greater than 300° F.
U.S. Pat. No. 6,582,819 discloses proppant composite composed of fillers (such as finely divided minerals, fibers, walnut shells, almond shells and coconut shells) bounded by a binder. Such composites, however, often are inadequate for downhole stresses and temperatures.
Thus, an improved composition of high particle strength at high temperature is needed for utilization in applications with high temperature and high pressure downhole conditions.
In particular, proppant composites are desired with significantly improved stress tolerance over the composites of the prior art.